“THE best time to turn a pay phone into a lending library is early on a Sunday morning, said John H. Locke, an Upper West Side architectural designer who may be the world’s leading expert on the subject.“There aren’t a lot of people out,” he said. “You can just go down, find a good booth, carry it out, latch it in. It takes seconds. And then just fill it up with books and let’s wait and see what happens.”